Remove securityGroupOverride validation for NLB

[h3poteto]

fix #17207

NLBs did not support security groups in the past. However, AWS now supports specifying a security group for an NLB. When kops creates a new cluster with an NLB, it also creates a security group for it by default.
Therefore, I believe we can now allow securityGroupOverride for NLBs in kops as well.

Please let me know if I'm overlooking anything or if there are any concerns with this approach.

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[hakman]

/ok-to-test

[justinsb]

I think this is a feature so should not delay releases? That said it seems pretty safe (as it's only validation IIUC), so I don't mind if we backport it to (say) 1.32.